Flux-modulation machines operate based on the so-called magnetic-gear effect. The modulator of the flux-modulation machine is one of the key elements. In this paper, a design method to obtain the distribution of modulators on split-pole flux-modulation machine is proposed to improve output performance. First, the operation principle of the flux-modulation machine is briefly discussed. Then, taking surface-mounted flux-modulation machines as examples, the air-gap flux density model is established by taking the distribution of the modulator into account. Besides, the formula of back-electromotive force of the flux-modulation machine is derived for both concentrated and distributed windings to determine the modulator distribution. Then, according to extreme equation, the optimal pole arc coefficient and distribution of the modulators can be obtained. Finally, the correctness of the proposed method is verified by both simulated and experimental results on a prototype machine.